1. Field of the Invention
The present invention relates to a CMOS image sensor and a method for fabricating the same. The present invention is suitable for a wide scope of applications, and is particularly suitable for enhancing a light-receiving capability of the image sensor.
2. Discussion of the Related Art
Image sensors are semiconductor devices for converting an optical image into an electrical signal and include charge-coupled devices and complementary metal-oxide-semiconductor (CMOS) image sensors. A general charge-coupled device includes an array of photodiodes that convert light signals into electrical signals. Disadvantages of a general charge-coupled device include a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photo process. In a charge-coupled device, integration of complementary circuitry such as a control circuit, a signal processor, and an analog-to-digital converter into a single-chip device is difficult. Thus, development of compact-sized or thin products is hindered. Examples of compact-sized or thin products that use such image sensors include digital still cameras and digital video cameras.
CMOS image sensors, on the other hand, adopt CMOS technology that employs a control circuit and a signal processing circuit as a peripheral circuit. CMOS image sensors also adopt switching technology that allows outputs to be sequentially detected using MOS transistors provided to correspond to a number of arrayed pixels, thereby detecting an image. Accordingly, a CMOS image sensor uses CMOS fabrication technology, i.e., a simple fabrication method using fewer photolithography steps, that enables an advantageous device exhibiting low power consumption.
In a CMOS image sensor of the related art, a photodiode is the active device that forms an optical image based on incident light signals by generating electrical signals according to the intensity and wavelength or color of incident light. In such a CMOS image sensor, each photodiode senses incident light and the corresponding CMOS logic circuit converts the sensed light into an electrical signal according to input wavelength. The photosensitivity of the photodiode increases as more light is able to reach the photodiode. In this instance, enhanced photosensitivity results from an increase in the levels of sensed light and corresponds to the light-receiving capability of the active device. One way of enhancing the photosensitivity of a CMOS image sensor is to improve its fill factor. The fill factor is the degree of surface area covered by the photodiodes versus the entire surface area of the image sensor. The fill factor is improved by increasing the area responsive to incident light, i.e., the photo-sensing portion. However, there is a limit to increasing the photo-sensing portion due to the required presence of the logic circuit portion.
Therefore, a device including a material exhibiting excellent light transmittance, such as a convex microlens having a predetermined curvature for refracting incident light, may be provided to redirect any light that may be incident on the image sensor outside the immediate area of the photodiodes. The convex microlens having the predetermined curvature concentrates or focuses the incident light on one or more of the photodiodes themselves. That is, the incident light, striking the surface of the convex structure of the microlens while in parallel to the optical axis of the microlens, is refracted by the microlens according to the curvature of the convex microlens. This refraction allows the incident light to become focused at a predetermined point along the optical axis. Accordingly, in a color image sensor, such a microlens may be provided over a color filter layer including red (R), blue (B), and green (G) color filter elements for passing the light of each wavelength or color to a photodiode area.
Meanwhile, CMOS image sensors are classified according to a number of transistors. For example, a 3T-type CMOS image sensor consists of one photodiode and three transistors, and a 4T-type CMOS image sensor consists of one photodiode and four transistors. An equivalent circuit and layout of a unit pixel of a general 3T-type CMOS image sensor are shown in FIG. 1 and FIG. 2, respectively.
Referring to FIG. 1, a CMOS image sensor of the related art comprises one photodiode PD and three NMOS transistors, including a reset transistor Rx, a drive transistor Dx, and a select transistor Sx. The cathode of the photodiode PD is commonly connected to the drain of the reset transistor Rx and the gate of the drive transistor Dx. The drain of the driver transistor Dx is connected to the source of the select transistor Sx, whose drain is in turn connected to a read circuit. With the anode of the photodiode PD grounded, a reference voltage VR is applied via a power line to the source of each of the reset transistor Rx and the drive transistor Dx. A reset signal RST is applied via a reset line to the gate of the reset transistor Rx, and a select signal SLCT is applied via a column select line to the gate of the select transistor Sx.
Referring to FIG. 2, an active area 10 is defined for each unit pixel of the CMOS image sensor of FIG. 1. The active area 10 includes a photodiode area 20 comprising the bulk of the active area 10, which is overlapped by gate electrode areas 30, 40, and 50 of the three NMOS transistors, respectively. The source/drain region of each transistor is formed by performing an ion-implantation process with respect to the active area 10. Power (Vdd) is applied to the source/drain regions of the reset transistor Rx and the drive transistor Dx. The source/drain region of the select transistor Sx is connected to the read circuit, and each of the gate electrodes 30, 40, and 50 is connected to external circuitry (not shown) via a corresponding signal line having a pad provided at one end.
FIG. 3 diagrams the incidence of light at the juncture of dissimilar media, i.e., media exhibiting different densities, including a first medium 1 having a refraction index n1 and a second medium 2 having a refraction index n2. Incident light travels in the first medium 1 at an angle of incidence θ1 and then enters the second medium 2, and travels in the second medium 2 at an angle of refraction θ2. According to an optical refractive law:n1×sin θ1=n2×sin θ2 n1/n2=sin θ2/sin θhd 1for a first medium (e.g., air) that is optically less dense than a second medium (e.g., a lens). Therefore, by using a lens material exhibiting a high refractivity to reduce total reflection occurring near a perimeter of a microlens, that is, light incident nearer the perimeter or in a peripheral area of the microlens, the angle of refraction increases for greater angles of incidence. Thus, the intensity of focused light is increased.
Referring to FIG. 4, illustrating a CMOS image sensor and a method for forming a microlens thereof according to the related art, at least one photodiode 12 for generating an electrical charge according to the intensity of an incoming light signal is formed in a predetermined surface of a semiconductor substrate 11. An insulating interlayer 13 is formed on the semiconductor substrate 11 including the photodiodes 12. A color filter layer 14 includes a plurality of color filters, such as red (R) green (G), and blue (B) color filters, and is formed on the insulating interlayer 13 to transmit light according to its wavelength or band. The color filters are arranged to correspond to the photodiodes 12. A planarizing layer 15 is formed on the color filter layer 14. At least one microlens 19 is formed on the planarizing layer 15 as an array of microlenses corresponding to the at least one photodiode 12. Each microlens 19 is formed by depositing, patterning, and reflowing a photoresist to have a convex structure with a predetermined curvature for concentrating light onto one photodiode 12 via a corresponding color filter of the color filter layer 14. Structural characteristics of the microlens 19, including its curvature and height, are determined in accordance with various device factors such as focal length, specific photodiode arrangement, color filter size, etc.
FIG. 5 diagrams the incidence of light at the juncture of dissimilar media to illustrate an optical phenomenon of light incident near a microlens perimeter A of the CMOS image sensor shown in FIG. 4. As shown, when light enters a medium 3, namely, a surface of the planarizing layer 15, which is a relatively sparse medium optically, from the more dense medium 2 at increased angles of incidence near the microlens perimeter A, the refracted ray tends to travel along the surface of medium 3. Thus, the total refracted light, incoming via the microlens 19, is inefficiently focused on the photodiode 12. This prevents an optimum quantity of light from reaching the photodiode.